Ionic liquid-channel charge-coupled device

ABSTRACT

An ionic liquid-channel charge-coupled device that separates ions in a liquid sample according to ion mobility characteristics includes a channel having an inner wall that has a matrix liquid disposed within. An insulating material surrounds the channel, and an introduction element introduces a liquid sample into the channel. The sample is preferably a liquid solution that has at least one ionic specie present in the solution. The device further includes a gating element that establishes at least one charge packet in the channel in response to an externally applied input sisal, and a transport element that induces the charge packet to migrate through the channel. The gate element can be a plurality of spaced-apart, electrically conductive, gate structures that are alternately disposable between a high voltage state and a low voltage state. The transport element further includes an application element that applies a variable voltage to the gating element. This application of voltage induces the charge packets to form under the gate structures and, when the voltage applied to an adjacent gate has a higher potential, induces the packet to migrate through the channel in that direction.

This invention was made with government support under contract No.F28-90-C0002 awarded by the Air Force. The government has certainlimited rights in this invention.

BACKGROUND OF THE INVENTION

This invention relates generally to charge coupled devices. Moreparticularly, it relates to a monolithic ionic liquid-channelcharge-coupled device (ILC-CCD) and methods of making the same.

The ability to quickly and to accurately analyze liquids to determinetheir constituents is of great general importance, and of particularimportance in the biomedical research industries. Over the years, avariety of techniques have evolved that can analyze a solution anddetermine its component parts. These techniques can be divided into twobasic categories. The first category includes fast and simpletechniques, which detect only a few specific constituents. Devices whichperform this technique include ion selectable electrodes, such as pHelectrodes. A drawback of this general category is that, although thetechniques are relatively fast and relatively simple to perform, theyonly detect specific sample constituents. Thus, a particular device istypically used to detect a relatively few, predetermined samplecomponents.

The second category includes techniques which can be used to detect abroader range of constituents. Such techniques include ion massspectrometers, gas chromatographs, and blood serum chemistry analyzers.A drawback of this general category is that the techniques requirerelatively large amounts of time and equipment to perform a completeanalysis of the solution constituents. In addition, relatively largeamounts of personnel, with their associated costs, are required tomonitor and perform the analyses.

Some prior art techniques have sought to address the above problems byincorporating integrated circuit (IC) theory and technology into thedevice design and fabrication. IC technology has led to the developmentof smaller, more cost effective devices that are capable of detecting abroader range of constituents. Moreover, by incorporating IC technology,techniques can now be employed that incorporate previously inaccessiblephysical principles.

One such physical principle that would be beneficial in analyzing liquidsamples is the physical principles employed by charge-coupled devices.Conventional charge coupled (CC) devices exist for generating a streamof electrons in response to an input signal, such as incident ambientlight. Consequentially, CC devices are employed in many types of imagingdevices, such as video recorders, camcorders, cameras and the like.Prior art CC devices usually consist of an array of polysilicon-silicondioxide capacitors, typically mounted on a substrate. The substratetypically has a top face to which an insulating layer of silicon dioxide(SiO₂) is applied. A series of gate electrodes are then mounted to thesilicon dioxide layer at various locations above the substrate. Eachgate electrode has an associated electrical connector that communicateswith an alternating current (AC) voltage source. The interface betweenthe silicon dioxide layer and the substrate top face forms an electronchannel. The channel defines the region in the CC device where electronsaccumulate.

During operation of the CC device, a voltage is applied to each gate ata selected occurrence rate, called a stepping frequency. Typically, theelectrons accumulate in packets under the gate electrode with thehighest potential, e.g., most positive. The electron packets can bemoved to the right or to the left of that gate by biasing the voltagespositively on either adjacent gate, and biasing the gate above thepacket negatively, such that the electrons move along the channel fromregions of low potential to regions of high potentials.

There still exists a need in the art for improved analytical devicesthat can detect the components of a liquid solution. One particularadvantageous device would employ the physical principles associated withCC devices. Additionally, a device that can relatively easily andrelatively quickly determine all the ionized constituents of a liquidsample would present a major improvement in the art. Moreover, a devicethat is relatively easy to manufacture, that is relatively low-cost, andthat is capable of detecting the presence of minute concentrations of asample component would also present a major improvement in the all.

SUMMARY OF THE INVENTION

The foregoing objects are attained by the invention, which providesmethods and apparatus for an ionic liquid-channel charge-coupled devicethat separates molecules in a liquid sample according to ion mobilitycharacteristics. According to a preferred embodiment, the deviceincludes a channel having an inner wall that has a matrix liquiddisposed within. An insulating material surrounds the channel, and anintroduction element introduces a liquid sample into the channel. Thesample is preferably a liquid solution that has at least one ionicspecie present in the solution. The device further includes a gatingelement that establishes at least one charge packet, e.g., aconstellation of similarly charged ions, in the channel in response toan externally applied input signal, and a transport element that inducesthe charge packet to migrate through the channel.

The gate element can be a plurality of spaced-apart, electricallyconductive, gate structures that are alternately disposable between ahigh voltage state and a low voltage state. The transport elementfurther includes an application element that applies a variable voltageto the gating dement. This application of voltage induces the chargepackets to form under the gate structures and, when the voltage appliedto an adjacent gate has a higher potential, induces the packet tomigrate through the channel in that direction. One of the gatespreferably applies a voltage to the insulating layer, which creates anelectric field orthogonal to the channel, thereby repelling ions thatmay accumulate along the channel walls.

The channel can be coupled to a reservoir that functions as a storagereceptacle for the matrix liquid and for the liquid sample. An electrodepad is preferably mounted in the reservoir and, in conjunction with aground electrode which serves as a current return electrode, allows theapplication of a voltage across the length of the channel.

In another preferred embodiment of the invention, a detector element isdisposed along the channel, which detects the presence of molecular andatomic species contained in the sample by measuring the changes in theconductivity of the channel as the species migrate across the detector.

According to another preferred embodiment, a device for separatingmolecular and atomic species in a matrix liquid can be fabricated byforming a first layer of a first insulating material upon a substrate,depositing a sacrificial structure of amorphous silicon upon the firstinsulating layer, and covering the structure with a layer of a secondinsulating material. An opening is then formed in the device and anetchant is introduced through the opening to remove the sacrificialstructure, thereby forming a buried channel. A plurality of gatestructures disposed at different locations along the channel receive anapplied voltage from a voltage source. This applied voltage induces themolecular species to migrate from gate-to-gate along the channel. Forexample, molecular species accumulate under the gates with the highestpotentials, and when an adjacent gate is biased more positively than thegate with the accumulated species, the species move in that direction.Hence, by biasing the adjacent electrodes either more positively or morenegatively, the species can be moved to the right or to the left of thatgate. Similarly, positively charged species will move to regions of thelowest potential.

In another preferred embodiment, before the sacrificial structure isdeposited on the insulated substrate, a layer of the second insulatingmaterial is deposited over the insulated substrate. Similarly, beforethe opening is formed in the device, another layer of the firstinsulating material is deposited over the second insulating material.Preferably, the first insulating material is silicon dioxide; the secondinsulating material is silicon nitride; and the etchant istetramethylammonium hydroxide.

The invention will next be described in connection with certainpreferred embodiments. However, it should be clear that various changesand modifications can be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example,various electronic detection units can be employed that detect thepresence of various ionic species in a sample solution. Additionally,the device can be formed from any number or combination of insulatinglayers.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following description and apparentfrom the accompanying drawings, in which like reference characters referto the same parts throughout the different views. The drawingsillustrate principles of the invention and, although not to scale, showrelative dimensions.

FIG. 1 is a cross-sectional view of an ILC-CCD according to a preferredembodiment of the invention;

FIGS. 2a-2c show the migration of a packet of ions through the channelof the ILC-CCD of FIG. 1, according to a preferred practice of theinvention;

FIG. 3 is a schematic circuit diagram of a preferred detection circuitused in conjunction with the ILC-CCD of FIG. 1;

FIG. 4 is a graph comparing the channel ion flux as a function ofstepping frequency for two selected ions:

FIG. 5 shows the separation of ions in the channel of the ILC-CCD ofFIG. 2, according to a preferred practice of the invention;

FIG. 6 is a graph illustrating the quantity of ion current generated inthe ILC-CCD of FIG. 1 as a function of both stepping frequency anddiffusivity;

FIG. 7 is a graphical illustration of the ion separating feature of theILC-CCD according to a preferred practice of the invention; and

FIG. 8 is an orthogonal cross-section of the ILC-CCD of FIG. 1illustrating the different material layers of the device according to apreferred embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts an ionic liquid-channel charge coupled device 50according to a preferred embodiment of the invention. The device 50includes a housing 51 having a channel 52 that is fluidly coupled to aninput reservoir 54, denoted by the black dashed lines. The reservoir 54contains a matrix liquid (not shown) that functions as a background ortransportation medium for various ionic species. The matrix solution canbe any analytic solution such as water, and is preferably glycerol. Anelectrode pad 56 mounts in the reservoir 54 and is connected to ground.A counter electrode 58 mounts to the housing bottom 60, and extends, ina first orientation, in a generally horizontal direction substantiallyparallel to the channel 52. Formed along the housing top 61 is a seriesof gate electrodes designated as IG, G1, G2 . . . Gn, OE1, BE and FG. Adetection unit 62 is capacitively coupled to the floating gate FG ordirectly to the channel 52.

Each gate electrode has an electrode contact mounted thereto that is inelectrical communication with an alternating current voltage source (notshown). Likewise, the counter electrode 58 has an electrode contact 59mounted thereto that communicates with the voltage source. Voltagesources suitable for use in the present invention are known in the artand commercially available.

A sample solution (not shown) containing various ionic species isintroduced into the channel 52 through the reservoir 54. The sample canbe introduced to the reservoir 54 by any suitable mechanical means.Preferably, the reservoir 54 is exposed to the ambient environment alonga top portion thereof, thereby facilitating the introduction of both thematrix liquid and the sample solution into the channel 52.

The electrode contacts preferably have low resistance and lowpolarization voltage. In a particularly preferred embodiment, thecontacts consist of an electrically conductive plating, e.g. silver orgold, disposed over an insulating material such as polymer. The housing51 is preferably formed using silicon chip technology, as described infurther detail below.

The device 50 separates ionic species contained in the liquid sample andheld in the reservoir 54. As shown in FIG. 2, the device 50, prior toanalyzing the sample constituents, clears the channel of eithernegatively or positively charged ions by biasing the gate electrodes andthe counter electrode 58 either positively or negatively, respectively.As illustrated in FIG. 2a. the gates and counter electrode 58 arepositively biased, effectively eliminating cations from the channel 52.Applying a potential lower than the originally applied potential on theinput gate IG' adjacent the reservoir 54', forms a packet 92 ofpositively charged ions, as illustrated in FIGS. 2b and 2c. The packet92 can be moved through the channel 52' and 52" towards the end oppositethe reservoir (output end) by negatively biasing adjacent gates GI',G2', G3' . . . Gn in succession, thereby producing an ion current to theright. FIG. 2c shows the ion packet 92 when stepped three gateelectrodes to the right, as well as the formation of a new ion packet 94under the input gate IG'. This method of transfer creates a "shiftingstaircase" potential profile under the gates, see FIG. 5. Those ofordinary skill will recognize that by biasing the input gate IGpositive, the ion current density in the channel is correspondinglyreduced since no new similarly charged ions diffuse into the channel 52.

Once a packet of ions is formed in the channel, the ion constituentsseparate in the channel according to their diffusion coefficients. Forexample, ions having relatively low diffusion coefficients havemigration velocities lower than ions having relatively high diffusioncoefficients. The ions with low diffusion coefficients migrate betweenthe regions between adjacent gate electrodes slower than ions withhigher diffusion coefficients. The voltages applied to the counterelectrode 58 and to the gate electrodes also creates an orthogonalelectric field which further induces the ions to migrate through thechannel 52. As a result, the ion packet separates into constituent partsas the packet migrates from gate-to-gate along the channel. The voltagesapplied to the gates operate at a frequency called the steppingfrequency which is defined as the number of voltage changes per second.At times, the stepping frequency operates at a sufficiently highfrequency such that ions with a low diffusion coefficient are preventedfrom traveling forward (away from the reservoir), thus becomingessentially immobilized. Accordingly, since the voltages applied to thegate electrodes are not inducing the ions to migrate through thechannel, the ion current for that particular constituent decreases alongthe length of the channel.

In a preferred embodiment, a suitable voltage is applied to a selectedgate electrode to repels ions that cohere to the channel walls. Duringtypical applications, the externally applied orthogonal electric fieldcreated by the applied voltage repels these ions from the channel wall.These ions interact with the ionic species migrating through the channel52 that are adjacent to the channel wall. As a result, the ionsconcentrated towards the center of the capillary travel at a velocityfaster than the ions along the wall surface, since the ions along thewall surface interfere with and ultimately decrease the migrationvelocity of ions traveling close to the channel wall.

The output electrodes OE1 and OE2 located at the end of the channel 52generate a potential well that allows ions traveling through the channelto accumulate at the output end. Preferably, the output electrodes arebiased at a constant voltage level to maintain a constant potentialwell. The floating gate FG capacitively couples the device 50 to thedetection unit 62, as described further below.

The detection trait 62 includes a detection circuit 64, as shown in FIG.3. The circuit 64 is a two-stage amplifier that includes feedbackresistor R2, and current sensing resistor R1. Transistor T1 ispreferably a floating gate MOSFET that is capacitively coupled to thefloating gate electrode FG, and hence the channel 52. The transistordrain 68 is coupled to a source voltage V_(D), and the transistor drain70 is connected to the drain 72 of transistor T2 and to the gate 75 oftransistor T3. Both the gate 73 and source 74 of transistor T2 areconnected to ground. By grounding the gate 73 of transistor T2, the gateto source voltage is pulled to zero, forcing the transistor to operatein the pinch-off region and effectively transforming the transistor T2into a load resistance. The drain 76 of transistor T3 is connected tovoltage source V_(D), and the source transistor 77 is connected to oneend of the current sensing resistor R1. The opposite end of the resistorR2 is connected to ground, and the output voltage 78 is measured acrossthe resistor R.

The feedback resistor R2 incorporates the signal from the source 77 oftransistor T3 to the gate 66 of transistor T1. The resistor R1 ensuresthat the transistor T1 is biased in the active region. The impedancevalue of resistor R1 is chosen such thin the time constant of thecapacitance of channel 52 and the resistor R1 is much larger than theexternally applied stepping frequencies. In other embodiments, theresistor R1 can be replaced with a switch which shorts in the channel52.

Again, with reference to FIG. 3, when no ion packet is positionedbeneath the floating gate FG that is capacitively coupled to transistorT1, no ion current is present to change the potential at the gate 66 ofT1, and the current through T1 does not change. As a result, the outputvoltage measured across resistor R1 remains essentially the same. Whenan ion packet is positioned beneath the floating gate FG by manipulatingthe adjacent gate voltages, the ion packet changes the floating gatepotential, which modulates the current at the gate 66 of transistor T1,thereby charging the current through T1. The drain current changeflowing from the drain 76 to the source 77 of transistor T3 creates anoutput voltage change across resistor R2. This output voltage can beconnected to a variety of display apparatuses, such as digital displayunits, LED read-outs and the like.

Alternatively, the detector can be an element that responds to changesin the channel 52 through capacitive coupling or the like.

In a preferred embodiment, the device 50 includes between 600 and 1000gate electrodes that extend along the length of the channel 52. Thedimensions of the channel 52, when fabricated, are preferably between 10and 500 nm high (equal to or less than the Debye length of the liquidmatrix), between 1 and 10 μm wide, and between 0.3 and 5.0 mm long.

The ion separating ability of the present invention is exemplified bythe graph depicted in FIG. 4, which shows a comparison of the ioncurrent (measured as the ion flux per step) in the channel 52 for twoselected constituents (Li⁺ ions and H⁺ ions) having substantiallydifferent diffusion coefficients. The ion currents are measured over arange of stepping frequencies, and are logarithmically graphed forcomparison purposes. As illustrated, at relatively low steppingfrequencies, the H⁺ ion concentration 82 and the Li⁺ ion concentration84 are essentially equal. The device 50, therefore, induces the ions totravel through the channel at approximately the same velocity, despitethe differences in diffusivity. However, as the frequency increases, thechannel 52 develops a substantially H⁺ ion current and a substantiallyreduced Li⁺ ion current. At still higher frequencies, the Li⁺ ioncurrent reduces to near zero. The Li⁺ ion current decreases withincreasing stepping frequency since the gate electrodes along thechannel switch at a selected rate which first impedes the forwardprogress of the ions, and then eventually reverses the migrationdirection of the ions. The dynamics of this ion migration and separationare depicted in FIG. 5.

FIG. 5 shows the separation of ions in the channel 52 as an ion packet92 is transferred through the channel 52 by varying the steppingfrequency of adjacent gate electrodes IG, G1, G2, G3 . . . Gn. Theshifting staircase potential produced by the stepping frequency formspotential wells 96 and 98 that can be repeated by varying the gateelectrode voltages in a selected sequence. The dashed line representsthe zero potential locations in the channel 52. In a preferredembodiment, the potential at the gate electrodes are repeated everythree gates, where each set of three gates is called a cell 90. Asillustrated, the ion packet 92 includes ions having different diffusioncoefficients, designated as fast ions F, slow ions S, and medium ions M.During a selected cycle, the ion packet 92 constituents separate as theyare transferred through the channel 52. When the stepping frequency ofthe cycle is high enough, the fast ions F travel forward, while the slowions S remain essentially stationary. Some of the ions with a relativelymedium diffusion coefficient M travel forward, while others remainsubstantially stationary. It will be readily apparent to those skilledin the art that the gate electrodes can be biased so as to cause theions to travel back into the reservoir (to the left), such that no ioncurrent is generated for any particular ionic species.

It is a significant feature of the ILC-CCD device of the presentinvention that constituents of an ion packet can be separated despitehaving diffusion coefficients within a narrow band, and withoutsuffering a corresponding loss in ion resolution. The ion packetconstituents can be separated by operating the device 50 at differentfrequencies, and preferably at three different frequencies. A preferredmethod is to clock the CC device 50 back at frequency f1 for N1 periods,forward at frequency f2 for N2 periods, and then back again at frequencyf3 for N3 periods. When the frequencies and the time periods areproperly adjusted, a net forward ion flux is created for a window ofdiffusivity. The net forward ion flux accumulates ions, e.g., cations,at the output gate electrodes OE1 and OE2 after each cell cycle. Theions continue to accumulate at the output until the back diffusion fluxof ions equals the net forward ion flux, e.g., pumping flux. Thus, forions within this window of diffusivity, their concentration increasesalong the length of the channel, while all other ions decrease to zero.

FIGS. 6 and 7 graphically illustrate the above "frequency sweeping"method where the ILC-CCD with elements 1 μm in length is repeatedlyoperated at three different frequencies. By way of example, the ILC-CCD50 is first operated at a frequency of 100 Hz at 10 clock periods insuch a manner as to produce a net ion current back into the reservoir54, pumping virtually all cations backward. The device is then clockedforward at a frequency of 1000 Hz for 100 clock periods, which producesa net forward ion current out of the reservoir. The ion packetconstituents include ions having relatively medium and relatively fastdiffusion coefficients. Then at the highest frequency of 1500 Hz, thedevice is again clocked backwards 112 cell periods, pumping the fastestions backward in the channel 52. The process is then continuallyrepeated. FIG. 7 compares the ion concentration in the channel againstthe diffusion coefficients of the ions during the frequency sweepingmethod. At 100 Hz, there is only a net concentration in the channel ofions with diffusion coefficients above 5×10⁻⁶, as illustrated by line100. The bold arrow represents the direction the device 50 was clockedduring this process. At the intermediate frequency of 1000 Hz, with thedevice now clocked forward, the ion flux increases with the diffusioncoefficient, as shown by line 102. Similarly, when the device is clockedat the highest frequency of 1500 Hz, the ion flux also increases withincreasing ion diffusivity, as shown by line 104, despite the ions beingpumped backwards. The net pumping flux for the three frequency sequenceis the weighted sum of the individual fluxes produced by each of thethree frequency operations. The fluxes are weighted according to thenumber of clock periods in which the frequency operates. Thus, a cationwith a particular diffusion coefficient, e.g., 1×10⁻ 5 cm² /s,accumulates at the output of the device, while the ions with higher andlower diffusion coefficients are pumped back into the reservoir. Theability to separate and concentrate at the output of the devicedifferent ions having different diffusion coefficients within a narrowrange, allows for the detection and identification of extremely smallamounts of ions. Those skilled in the art will also recognize that thefrequency sweeping can occur in the reverse direction.

FIG. 7 graphically illustrates the ability of the present invention toidentify constituents of a sample solution that are within a relativelynarrow window of diffusion coefficients. The device 50 (as shown in FIG.2) pumps ions present in the channel 52 to the right, until the ionconcentration at the left (near the reservoir 54) of the channel 52increases to where diffusion of ions back towards the reservoir from theoutput end of the channel equals the pumping current. By employing theabove-mentioned stepping frequencies and clock periods, the maximumconcentration of ions at the channel output occurs at a diffusivity of1×10⁻⁵ cm² /s, indicated by bold arrow 114. At this diffusivity, thereis approximately a 10% increase in the ion concentration between eachgate electrode at the output end of the channel. Ions having diffusivitylevels approximately 10% higher or lower than the ion having adiffusivity of 1×10⁻⁵ cm² /s are pumped back towards the reservoir, andeliminated from the channel 52. The bold arrows 110 and 112 show theforced migration of ions with difference diffusivities back towards thereservoir 54. By varying the stepping frequency of the gate electrodes,different ions can be concentrated at the output end. This significantfeature allows the ILC-CCD to reject alias compounds that have diffusioncoefficients substantially similar to that of a desired analyte. Theincreasing ion concentration along the length of the channel, inconjunction with the ion separation that occurs, allows the detectionunit 62 to detect and identify a particular ion within the range ofcoefficients, which would otherwise be undetectable without thisconcentrating ability.

In accordance with the present inventive device, ions accumulate in thepotential well formed beneath the output electrode, prior to transferbetween the output electrodes. By way of example, and returning to FIGS.2 and 6, ions accumulate beneath output electrode OE1 prior totransferring the ions to the adjacent electrode OE2. Accumulating theions in a parcel beneath the electrode OE1 and then expeditiouslytransferring the parcel to the adjacent output electrode OE2, by biasingthe electrode OE2 more negatively, avoids individually pulling the ionsacross the region between the two electrodes over a longer period oftime. An advantage of this approach is that it effectively eliminateslow frequency noise problems that are characteristic of charge-coupleddevices and detection circuits when operating at these lowerfrequencies. This advantage manifests itself in better sensitivity tovery low ion concentrations by virtually eliminating the problem of"popcorn" and 1/f voltage noise.

FIG. 8 illustrates the various layers and materials that are utilized toform the housing 51 of the device 50 of the present invention. Thehousing is formed by depositing a layer of polysilicon 202, preferablyphosphorus-doped, over the substrate 221 by a fabrication technique,such as chemical vapor deposition. The layer 202 preferably forms thecounter electrode 58, and the substrate is preferably a semiconductorwafer. The polysilicon layer is electrically and ionically insulatedfrom the matrix liquid and the sample solution by a layer ofelectrically insulating material 204, and a layer of silicon nitride(Si₃ N₄) 206, which is also deposited on the insulating material 204 bychemical vapor deposition or like techniques. Both layers have athickness of 50 nm. The substrate is preferably insulated from thepolysilicon layer 202 by insulator 200, which is preferably a 2 μm thicklayer of silicon dioxide.

A sacrificial structure 210, such as amorphous silicon, is positioned onthe silicon nitride layer 206. If more than one channel is desired, thena series of sacrificial structures can be placed along the layer 206. Asecond layer 208 of silicon nitride is formed over the structure 210,and a second layer of insulating material 212 is deposited over thesilicon nitride layer 208. The silicon nitride layers 206 and 208 formthe upper and lower halves of the channel 50. Again, a layer ofpolysilicon 214 is deposited on the insulating layer 212, and ispatterned to form the gate electrodes IG, G1, G2, G3 . . . Gn. Finally,a third encapsulation layer of the silicon dioxide 216, is depositedover the gate electrodes.

The channel 52 is created by aperturing the assembly 220 to form anopening (not shown) that communicates with the sacrificial structure210. An etchant, preferably tetramethylammonium hydroxide, is introducedthrough the opening and completely removes the structure 210 to form thechannel 52, without harming the silicon nitride layers 206 and 208.

The channel 52 formed in the assembly 220 is preferably elongate inshape. However, whenever it is desired that the overall length of thechannel exceed the diameter of the silicon wafer, the channel length canbe increased by forming the sacrificial structure in a serpentine,spiral, or other like configuration.

Another advantage of the present invention is the low voltage levelsrequired to facilitate separation of the ion packet into constituentparts. Low voltage levels can be employed by the device 50 since thegate electrodes are spaced relatively close together. In a preferredembodiment of the invention, the voltage difference between adjacentgate electrodes is preferably between 5 V and 60 V, and more preferably,20 V. The separation between adjacent electrodes is preferably between0.1 μm and 10 μm, and more preferably about 1 μm. The gate length ispreferably between 0.1 μm and 50 μm, and more preferably about 1 μm.

Having described the invention, what is claimed as new and desired to besecured by Letters Patent is:
 1. A device for separating molecules andcations in a liquid sample, the device comprising:a channel having adefined length and height and an inner wall, and surrounded by aninsulating material, and having a matrix liquid disposed within thechannel; introduction means for introducing a liquid sample into thechannel; gating means, coupled to the channel, for establishing at leastone charge packet in the channel in response to an applied input signal;and transport means the moving the charge packet along the channel. 2.The device of claim 1 wherein the channel height is equal to or lessthan the Debye length of the liquid matrix.
 3. The device of claim 1where the gate means further comprises a plurality of spaced-apart,electronically conductive, gates coupled to the channel and alternatelydisposable between a high voltage state and a low voltage state.
 4. Thedevice of claim 3 wherein at least one gate electrode applies a voltageacross the insulating material surrounding the channel, therebyrepelling ions away from at least one surface of the channel wall. 5.The device of claim 1 wherein the transport means further comprisesmeans for applying a voltage varying signal to the gating means.
 6. Thedevice of claim 1 wherein the device further comprises at least oneelectronic detector disposed along said channel to detect the presenceof an ionic species by measuring the conductivity of the channel.
 7. Thedevice of claim 1 wherein the channel has at least one end in fluidcommunication with a reservoir that contains the matrix liquid.
 8. Thedevice of claim 1 wherein the channel is formed upon a semiconductorwafer.
 9. The device of claim 3 wherein the gates are formed frompolysilicon.
 10. The device of claim I wherein the insulating materialis formed from an assembly which includes an inside layer and an outsidelayer, wherein the inside layer is silicon nitride and the outside layeris silicon dioxide.